1. Field of the Invention
The present invention relates to the field of measuring and manufacturing optical surfaces using an interferometer apparatus. In particular the invention relates to processing an optical element having a spherical surface.
2. Brief Description of Related Art
The optical element having the optical surface is, for example, an optical component such as an optical lens or an optical mirror used in optical systems, such as telescopes used in astronomy, or systems used for imaging structures of a mask (“reticle”) onto a radiation sensitive substrate (“resist”) in a lithographic method. The success of such an optical system is substantially determined by the precision with which the optical surface can be machined or manufactured to have a target shape. In such manufacture it is necessary to compare the shape of the machined optical surface with its target shape, and to determine differences between the machined and target surfaces. The optical surface is then further machined at those portions where differences between the machined and target surfaces exceed e.g. a predefined threshold.
Interferometric apparatuses are commonly used for high precision measurements of optical surfaces. Examples of such apparatus are disclosed in U.S. Pat. No. 4,732,483, U.S. Pat. No. 4,340,306, U.S. Pat. No. 5,473,434, U.S. Pat. No. 5,777,741, U.S. Pat. No. 5,488,477. The entire contents of these documents are incorporated herein by reference.
A conventional instrument and method for qualifying an optical element having a spherical optical surface will be illustrated with reference to FIG. 1 below. An interferometer apparatus 1 is of a Fizeau interferometer type having an optical axis 3 and a laser light source 5 such as a Helium-Neon-gas laser, emitting a laser beam 7. A microobjective 9 collimates laser beam 7 onto a pinhole of a spatial filter 11 such that a diverging beam 13 of coherent light having spherical wavefronts originates from the pinhole of spatial filter 11. An interferometer optics 15 having plural lenses 17,18,19,20,21 transforms diverging beam 13 into a converging beam 23 such that a crossover of converging beam 23 is formed in a region 25 on optical axis 3. Wavefronts in converging beam 23 are substantially spherical wavefronts.
Lens 21 is the focusing lens of the interferometer optics which is closest to crossover 25 of converging beam 23. A concave surface 27 of focussing lens 21 forms a Fizeau surface of Fizeau interferometer 1. Concave surface 35 has a substantially spherical shape with crossover 23 as its center. Surface 27 is partially reflective, and spherical wavefronts of converging beam 23 are partially reflected from surface 27 such that they travel back in the beam path of interferometer apparatus 1 and are transformed to substantially spherical converging wavefronts by interferometer optics 15. A partially transmissive mirror 29 is arranged in the beam path of beam 13 such that wavefronts reflected from Fizeau surface 27 are imaged by a camera lens 31 onto a light sensitive substrate of a camera 33.
The optical element 35 to be manufactured has a convex spherical surface 37 to be qualified. Element 35 is arranged in the beam path of converging measuring light beam 23 such that surface 37 faces interferometer optics 15 and such that a center of curvature of convex surface 37 substantially coincides with crossover 25. Spherical wavefronts of measuring light beam 23 are reflected from surface 37 and travel back through interferometer optics 15 and are imaged onto camera 33. On camera 33 the wavefronts reflected back from reference surface 27 and the wavefronts reflected back from surface 37 under test are superimposed and form interference fringes detected by camera 33. From a measurement of such interference fringes deviations of surface 37 from its target spherical shape may be determined. Based on such determination surface 37 may be machined for better conforming to its target shape.
An opening ratio k may be defined for surface 37 having a diameter D and a radius R as
      k    =          1              2        ⁢                                  ⁢        sin        ⁢                                  ⁢        α              ,wherein α is an opening angle and satisfies D=2Rsinα. This opening ratio is often referred to as f-number F/#.
It appears from FIG. 1, that for small values of the opening ratio k diameters of lenses 17,18,19,20,21 of interferometer optics 15 have to be substantially higher than diameter D of spherical surface 37 under test. Further, the interferometer optics must comprise a considerable number of plural lenses having spherical surfaces for providing the beam 23 of measuring light having wavefronts which do not deviate from the spherical shape to an extent such that the measurement precision is reduced by such deviation. Manufacture of interferometer optics having plural lenses of a high diameter with the necessary precision is not only expensive but also demanding from its optical design.